Organic photovoltaic (OPV) cells offer several advantages over the traditional inorganic photovoltaic cells. OPV cells can be made with flexible substrates, they are lightweight, and amenable to production by inexpensive techniques such as spin-coating, doctor blading, and printing [1]. OPV cells could even be incorporated into clothing. The unique set of advantages makes organic solar cells (OSCs) desirable in several areas where the heavier, inflexible inorganic cells are difficult to work with. For example, military gears could use OPV cells to reduce the weight of batteries carried by soldiers in the field; tents with OPV cells on the roof could become portable headquarters with a lightweight power supply built into the roofs; and countless portable electronic devices like cell phones, PDAs and MP3 players could regenerate some of their battery while in use outdoors, prolonging use between charges. However, the only holdup is that the power conversion efficiency (ηp) of OPV cells is currently too low for them to be used commercially. It is estimated that if an organic solar cell could reach ηp=10%, which seems possible [6], it will likely become commercially viable [1].
Approaches exist for increasing the power conversion efficiency of OPV cells. Several breakthroughs have yielded the power conversion efficiencies of OPV cells relatively close to what they need to be to find a niche in the market [2-5]. It is demonstrated that bulk-heterojunction (BHJ) solar cell design may improve the power conversion efficiency. In a BHJ solar cell, a donor polymer such as poly(2-methoxy-5-(3′,7′-dimethyl-octyloxy))-p-phenylene vinylene (MDMO-PPV) or poly(3-hexylthiophene) (P3HT) and an acceptor material such as [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) are combined in solution and together spin-coated to form a phase-separated blend on the transparent conductive anode, usually tin-doped indium oxide (ITO) anode. Fabrication is completed by depositing a metal such as aluminum as the cell cathode. Currently, BHJ solar cells have achieved the power conversion efficiency ηp around 2.5% to 5% [7, 8, 14].
The power conversion efficiency ηp of a solar cell is defined as follows:
                              η          p                =                                            V                              o                ⁢                                                                  ⁢                c                                      ⁢                          J                              s                ⁢                                                                  ⁢                c                                      ⁢            F            ⁢                                                  ⁢            F                                P            o                                              (        1        )            where Voc is the open-circuit voltage, Jsc is the short-circuit current, FF is the fill factor, and Po is the incident light intensity on the cell during testing in units of watts per area. The definition of the power conversion efficiency shows that increasing the open-circuit voltage (Voc), the short-circuit current (Jsc) and the fill factor (FF) will all lead to enhance the power conversion efficiency.
The origin of the open-circuit voltage has been debated in the literature [9-13]. However, it is generally thought that the open-circuit voltage Voc originates from the energetic difference between the highest occupied molecular orbital (HOMO) of the donor material and the lowest unoccupied molecular orbital (LUMO) of the acceptor material of the BHJ solar cells. This difference is the theoretical maximum of the open-circuit voltage Voc. However, in practice, the open-circuit voltages Voc achieved are 300 mV or more less than the maximum value. This loss in the photovoltage is broken down by Scharber et al. [6] into a 100 mV loss attributed to the fact that the photocurrent in the BHJ solar cells is mainly field-driven and a 200 mV loss caused by dark current. If some significant part of this 300 mV loss in the open-circuit voltage Voc could be recovered, the overall ηp of the solar cell would increase significantly.
In a BHJ solar cell, both the donor polymer and the acceptor molecule are touching both electrodes, interfacial effects probably limit realization of the maximum theoretical open-circuit voltage Voc. For example, electrons in the PCBM may be formed at an interface very close to the ITO anode, which typically collects the holes. There may, however, be some small flow of electrons from the PCBM near the ITO anode/organic interface to the ITO anode. Although this leakage current flows against the built-in electric field in the BHJ solar cell, it is energetically favorable for an electron in the HOMO level of PCBM (about 4.1 eV) to transfer to the ITO anode (workfunction≈4.7 eV). Any electrons transferred to the ITO anode would essentially recombine with holes and reduce the working voltage of the BHJ solar cell. In this way, the open-circuit voltage Voc, or the voltage that is required to counter any photovoltage created by the solar cell and reduce the photocurrent to zero, would also be reduced. A similar effect should take place at the cathode if holes are produced in the HOMO of the polymer very near the Al cathode.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.